1. Field of the Invention
The present invention relates to a brushless motor having permanent magnets received therein, and more particularly, to a motor having permanent magnets in which the magnets are disposed so as to have uniform magnetic flux and a rotor for decreasing magnetic flux leakage and enhancing the usefulness of the magnetic flux is adopted.
2. Description of the Related Art
Brushless motors are gaining in popularity as the home appliances or industrial equipment becomes to have high performance, lighter weight, and more compact. In addition, motor control technique is being widely used due to the development in the areas of semiconductor technique or substances, which enhances the reliability of brushless motors.
FIG. 1 shows the structure of brushless motors, FIG. 2 shows the waveform of the current flowing at each phase shown in FIG. 1, FIG. 3A is a vector diagram illustrating the magnetic field generated from the current, and FIGS. 3B to 3E are vector diagrams illustrating the magnetic field generated according to each angle at which a rotor rotates.
As shown in FIG. 1, around a rotor 1 having two poles N and S, stator coils Lu, Lv, and Lw at which three phase current flows are arranged at 120.degree.. In addition, one end portions of the stator coils Lu, Lv, and Lw are connected to a voltage supply source Vcc, respectively, and the other end portions of the stator coils Lu, Lv, and Lw are connected to switching elements Qu, Qv, and Qw, respectively. Switching elements are turned on or off according to the control of a driving controller 2, thereby conducting current to the coils.
As an example of using 180.degree. conducting method, if it is assumed that the current flows each phase as shown in FIG. 2, the magnetic field is emitted at a direction from the winded coils toward rotor 1. Since the coils are disposed at 120.degree., respectively, the magnetic field emitted therefrom is also at 120.degree., as shown in FIG. 3A.
As shown in FIG. 3B, current flows at coils Lu and Lv when an electrical angle is 0.degree.. That is, when the control signal from driving controller 2 turns the switching elements Qu and Qv on, the current flows at coils Lu and Lv, and magnetic fields Mu and Mv are generated. Then, a combined magnetic field MT1 is formed, and rotor 1 rotates in clockwise direction as shown in FIG. 3B, being affected by the combined magnetic field MT1.
When the electrical angle exceeds 60.degree. as shown in FIG. 2, the current at coil Lv is cut off, and generating the magnetic field Mv which makes rotor 1 to be rotated in counterclockwise direction is restricted. In other words, the current flows only at the coil Lu, which generates only the magnetic field Mu. Here, rotor 1 rotates as shown in FIG. 3C, and magnetic filed Mu vector faces the center of N-pole, thereby rotating rotor 1 at the same direction.
When the electrical angle exceeds 120.degree. as shown in FIG. 3D, the current is provided for coils Lu and Lw, and magnetic fields Mu and Mv are generated. Then, a combined magnetic field MT2 is formed, and rotor 1 rotates, being affected by the combined magnetic field MT2. When the electrical angle exceeds 180.degree. as shown in FIG. 3E, rotor 1 rotates in counterclockwise direction. At such a state, the current for generating magnetic fields Mu and Mv that make rotor 1 to be rotated in counterclockwise direction is cut off, and the current is provided only for coil Lu so as to generate magnetic field Mw for rotating in clockwise direction.
As shown in FIGS. 3A to 3F, the magnetic field generated at the stator coils appropriately controls the current of each phase, to thereby form a rotating magnetic field. The rotor rotates according to the rotating magnetic field.
The rotating magnetic field of brushless motors is made up of the combined magnetic field of each phase. Therefore, the size of the rotating magnetic field is not regular every moment, and the torque generated by the rotating magnetic field is not uniform, which causes a ripple. In addition, the combined magnetic field interacts with the permanent magnets of the rotor according to the structure of the rotor. However, if the combined magnetic field that passes through the interior of the rotor aggregates densely at a specific portion, the magnetic reluctance increases. If the rotor has an inappropriate structure, the magnetic flux generated from the permanent magnet leaks to the adjacent magnets, which degrades a motor efficiency.
FIG. 4A shows an example of the iron sheet used for a conventional brushless motor, and FIG. 4B is a section view showing a motor to which the rotator having a series of iron sheet is applied.
To reduce the core loss generated from rotor 1, a core 40 where a series of iron plates made up of a silicon are deposited is used for rotor 1. Accordingly, rotor 1 in section view is shaped same as the core. Core 40 has at a center thereof a hole 41 into which a rotating shaft is to be pressed and fixed, holes 42 into which bolts are inserted so as to fix core 40, and slots 44 into which permanent magnets are to be received oppositely to each other from the rotating shaft. The magnetic flux generated from the permanent magnets received in slots 44 passes through the gap between rotor 1 and a stator 45 and flows toward a teeth 46 of stator 45. In other words, as shown in FIGS. 4B and 5, the magnetic flux generated from stator 45 travels teeth 46, core 40, and the permanent magnets received in slots 44, and travels again core 40, permanent magnets, and teeth 46, to thereby form a closed loop.
However, since the outer surface of the core is shaped as a circle centering from the shaft of the rotor, the portion A of the magnetic flux from the permanent magnets directly flows toward the opposite magnetic poles of the permanent magnets, as shown in FIG. 5. That is, all of the magnetic flux from the end of the permanent magnets received in the slots does not flow toward to the stator, which means the magnetic flux partially leaks as shown in portion A of FIG. 5. This occurs because the core at the end of the permanent magnet is shaped as a circle, which provides a sufficient core area for the flowing of the magnetic flux.
The leakage magnetic flux is useless since it does not affect to the rotation of the rotor, which degrades a motor efficiency.